1. Field of the Invention
This invention relates to a method of preparing trans-3,4-disubstituted-.gamma.-lactones. The lactones are contained, for example, in fragrance ingredients and insect pheromones.
2. Description of the Related Art
Many trans-3,4-disubstituted-.gamma.-lactones having substituents such as an alkyl group or an alkenyl group at the 3 and 4 positions such that they are in positions trans to each other, are present in the natural world. For example, they are important fragrance ingredients contained in whisky, cognac, etc. They are also utilized as pheromones in the insect world.
Trans-3,4-disubstituted-.gamma.-lactones are generally present in the optically active form. Different features such as biological activity, etc., are often recognized between two optical isomers of the lactones. Therefore, it is extremely important to develop a method for synthesizing optical isomers of trans-3,4-disubstituted-.gamma.-lactones in chemically pure form.
One of the trans-3,4-disubstituted-.gamma.-lactones as described above is lactone (I) of the following general formula (I): ##STR1## wherein R and R.sup.1 are general alkyl or alkenyl groups.
Conventionally, various methods have been developed to prepare the lactone (I). Examples of the conventional methods include the following:
1) A method utilizing ribonolactone as a starting material (Tetrahedron, Vol. 43, 4497-4506 (1987)).
2) A method utilizing asymmetric synthesis (Agnew. Chem. Int. Ed. Engl., Vol. 28, 69-71 (1989)).
However, method 1) is not practical, because, ribonolactone which is the starting material is expensive, and the reaction steps in method 1) are complicated.
Method 2) has the following disadvantages: one is that it is necessary to use a specific reagent such as (-)-sparteine, and another is that the optical purity of products obtained via method 2) is not sufficiently high.